On July 21, 1969, the Apollo 11 astronauts lifted off from the moon’s surface, taking with them
47.5 pounds of rocks. When analyzed, those rocks and others collected on later Apollo missions were
found to contain several minerals that had never been found on Earth.

One of those new minerals was given the name armalcolite, based on the first letters of the
Apollo 11 astronauts’ last names: Armstong, Aldrin and Collins.

Armalcolite is iron magnesium titanium oxide. It occurred in fine-grained lunar basalts as small
rectangular crystals often rimmed with ilmenite, which is iron titanium oxide. In 1975, it was
discovered in Montana, in a kind of igneous rock called lamproite.

Similar to kimberlite, lamproite comes from the mantle, deep below ground. Also like kimberlite,
some Earth lamproite contains diamonds.

Another lunar mineral was pyroxoferroite, a silicate that contains iron, calcium, magnesium and
manganese that occurs as crystals as long as 1 mm in lunar basalts and gabbros.

It was discovered in Japan around 1980.

Another new mineral was named tranquillityite after the Sea of Tranquility, the location where
the Apollo 11 astronauts landed. It was later found at all of the Apollo landing sites.

Tranquillityite is composed of silicon, zirconium, titanium and iron. It is a red mineral that
occurs as small laths or sheaf-shaped crystals in lunar basalts.

Tranquillityite was commonly associated with other zirconium-containing minerals such as
zirconolite and baddeleyite, and with phosphate-containing minerals such as apatite (the
calcium-phosphate mineral in your bones and teeth) and merrillite.

Since its official naming in 1971, tranquillityite has been discovered in other Apollo samples
and in lunar meteorites, which are moon rocks blasted to Earth by the impacts of larger meteorites.
But it had yet to be found in Earth rocks.

Until now.

As reported in the January issue of the journal
Geology, the mineral was found at six sites in western Australia in an igneous rock called
dolerite.

Dolerite is chemically equivalent to basalt but has larger crystals because it cooled slowly
below ground, where other rocks insulated it. The Australian samples were found in dikes and sills,
thin sheets of rock that as magma melted their way into pre-existing rock.

As an added bonus, the Australian samples of tranquillityite could be dated using lead-207 and
lead-206. Those igneous rocks cooled and crystallized 1,064 million years ago, and because the
rocks they melted into had to have been there first, those rocks are even older.

Those rocks were thought to have been much younger. The researchers suggest that tranquillityite
might be a relatively widespread, albeit minor accessory mineral in similar rocks elsewhere on
Earth.

Tranquillityite is the last formerly exclusive lunar mineral to be found on Earth. Maybe it is
time to go back to the moon to find some more.

Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.